1. Field of the Invention
The present invention relates to a method of inspecting the mura defect, an apparatus for inspecting the mura defect, and a method of producing a photomask.
2. Description of the Related Art
A photomask used in pattern formation in a process of producing a semiconductor device, an imaging device, or a display device has a configuration in which a opaque film made of chromium or the like is partly removed away to be formed into a desired pattern on a transparent substrate such as a glass plate.
The photomask is produced in the following method. The opaque film is formed on the transparent substrate, and a resist film is formed on the opaque film. Then, a predetermined pattern is exposed onto the resist film by conducting a writing process with using an electron beam or a laser. Thereafter, written portions and unwritten portions are selectively removed away to form a resist pattern. The opaque film is etched with using the resist pattern as a mask, to form an opaque film pattern. Finally, the residual resist is removed away, and a photomask is obtained.
Lithography apparatus which applies a writing process directly to the resist film by means of scanning with the electron beam or the laser are roughly classified into a raster scan system and a vector scan system. In the raster scan system, the beam (the electron beam or the laser) scans over the whole writing area, and, when the beam reaches a pattern portion, the beam is turned ON to draw a pattern. The beam scans in the Y-direction with a constant scan width. When the scanning operation in the Y-direction is ended, the beam is fed in the X-direction. These operations are repeated to scan the whole writing area.
On the other hand, in the vector scan system, in each of plural scanning areas (writing units), a beam scans only over a portion in which a pattern is to be formed. When a scanning operation for one pattern is ended, the beam is turned OFF, and then moved to the next portion in which a pattern is to be formed, in the same scanning area. The beam is again turned ON, and scans over the portion. When all patterns in the scanning area have been written, the beam is moved to the next scanning area by movement of an X-Y table.
When any of the above-described writing systems is employed, writing joints exist due to the beam scan width or the beam diameter, and there arises a problem in that an error due to a writing failure such as a butting error occurring in such joints is periodically caused in each of the writing units.
In a member having a repetitive pattern which is regularly arranged in accordance with a pixel pattern, for example, a photomask for producing an imaging device such as a solid-state imaging device (e.g., a CCD, a CMOS, etc.), a display device such as a liquid crystal display panel, or like image device, particularly, periodic errors, which are caused by a writing failure as described above, occur on a regular repetitive pattern. Even when each of the errors has a size which is so small as to be invisible in micro scale (for example, several tens of nm), the errors result in a continuous linear error become to be visible. As a result, a mura defect such as mura in sensitivity or that in display occurs to cause a problem in that the performance of the device is lowered. A regular error which occurs unintentionally in a regularly arranged pattern as in the above example is referred to as “mura defect.”
Conventionally, the pattern demension inspection, the pattern defect inspection, and the particle inspection are usually conducted as inspections for ensuring the quality of a photomask. However, these inspections are conducted for inspecting local defects on a photomask, and hence hardly detect a defect such as the mura defect or a defect which is recognized only in a wide area on the photomask. Therefore, the mura defect is detected by a visual inspection which is subjective inspection.
On the other hand, for example, Japanese Patent Unexamined Publication JP-A-10-300447 discloses an apparatus for inspecting the mura defect in an image device substrate (for example, a TFT liquid crystal substrate). In the apparatus for inspecting the mura defect, the surface of a substrate is illuminated with light, and scattered light from an edge portion of a pattern formed on the surface is observed, thereby detecting the mura defect.
In such a visual inspection of the mura defect, however, inspection results are dispersed depending on the worker conducting the inspection. Because of this reason and the like, the mura defect of the photomask cannot be accurately detected. As a result, there is a problem in that the yield of devices is lowered.
In a device produced with using the photomask, it is considered that the mura defect of the device is caused not only by the photomask but also by other reasons such as stitch exposure of the photomask. In the apparatus for inspecting the mura defect disclosed in the JP-A-10-300447, it is difficult to identify the cause of a detected mura defect.
The apparatus for inspecting the mura defect disclosed in the JP-A-10-300447 can detect the mura defect. However, the apparatus cannot qualitatively evaluate the mura defect, and, for example, cannot differentiate the mura defect which requires an inspection from that which does not require an inspection, or quantitatively evaluate the mura defect on the basis of the intensity of the defect.